1. Field of the Invention
The present invention relates generally to a measuring head and, more specifically, to a measuring head implementing light guides for the planar measurement of a sample.
2. Description of the Related Art
Conventional measuring heads typically include an optical projection means to illuminate a measuring surface and optical measuring means for measuring the light reflected. An example of such a measuring head is described in U.S. Pat. No. 4,078,858, which comprises an incandescent lamp, which in order to produce a measuring spot is associated with a collimator lens and a deviation mirror, together with a further collecting lens. The light reflected by the measuring spot at an angle of 45 degrees arrives in the optical measuring means, which comprises a spherical annual mirror and a flat spherical mirror.
One application of such conventional measuring heads is described in an apparatus to determine the optical density of photographs described in U.S. Pat. No. 3,244,062, which comprises a measuring head with an optical projection means, which images the light of an incandescent lamp on the photographic material and an optical measuring means which captures the light re-scattered essentially perpendicularly from the photographic surface by means of a deviating mirror transversely to the optical axis of the optical projection means and exposes the inlet aperture of a photoelectron multiplier with the aid of a lens. The optical projection means makes it possible to expose the measuring spot, at an angle of incidence of essentially 45 degrees on all sides, to the light originating in the incandescent light in the measuring head. The optical projection means correlated with the incandescent lamp contains a spherical annular mirror, which surrounds the incandescent lamp, with the filament of said lamp extending along the optical axis of the spherical annual mirror. The light of the lamp passing from the spherical annular mirror parallel to the optical axis of the optical projection means is focused by means of a simple annular mirror onto the measuring surface. The quality of the optical projection means and the optical measuring device of the known apparatus is adequate for simple density measurements in the determination of optical reflection properties, however is not adequate for detailed spectral analysis of the light reflected. Such conventional measuring heads convey large amounts of stray light and crosstalk in a reflected signal and typically require high-power light sources because of the loss of light in the optical cavity due to scattering.
In addition, conventional measuring heads which do not contain light sources within their housings require very specific alignment with respect to an outside light source for the collection of light from the light source to ultimately illuminate a sample. Such specific alignment procedures may prove to be very difficult and time consuming.